The present invention relates in general to an apparatus and method for the continuous lyophilization, also known as freeze-drying, of a material by sublimation under substantially atmospheric pressure.
Lyophilization is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. A common application for the lyophilization of perishable material include pharmaceuticals and food products. Lyophilization overcomes two of the common problems associated with conventional drying of material by evaporation. The first problem is the inherent difficulty of removing water completely using evaporation because most of the water is not directly exposed to the air. Second, the liquid water involved in the evaporation process significantly changes the shape, texture and composition of the material. This is a particular disadvantage in the process of drying material that it intended to be revitalized later.
The fundamental principle in lyophilization is sublimation which is an endothermic phase transition that occurs at temperatures and pressure below the triple point. At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases, the transition from the solid to the gaseous state requires an intermediate liquid state. As shown in the phase diagram of FIG. 1, the boundary between gas and liquid runs from the triple point 10 to the critical point 20. Freeze drying (arrow 30) brings the system around the triple point, avoiding the direct liquid to gas transition seen in ordinary drying (arrow 40). Sublimation occurs when the partial pressure of the compound or element in the surrounding atmosphere is less than the vapor pressure of the compound or element in the material being lyophilized. Various apparatus and methods for lyophilization of material are known from U.S. Pat. Nos. 5,269,077; 4,561,191; 4,104,807; 3,731,392; 3,484,946; and 3,462,849, the disclosures of which are incorporated herein by reference and copies enclosed herewith.